U.S. Pat. No. 4,118,712 discloses the use of switchable current sources for digital-to-analog conversion of a counter value. The switched currents are proportional to the binary values of the signals used for switching. The switched currents are summed to produce a voltage that is proportional to the logarithm of the binary value being converted.
U.S. Pat. No. Re. 29,619 also shows digital-to-analog conversion using switchable current sources and current summing. Another constant current switching circuit is shown in U.S. Pat. No. 3,787,734 which also uses a constant voltage reference source.
U.S. Pat. No. 4,282,478 shows the sensing of a current through the load using a voltage drop resistor in conjunction with a threshold detector for controlling the current.
U.S. Pat. No. 4,277,696 shows a three-transistor, three-terminal switching circuit using an inverted Darlington pair to provide thyristor switching action.
These references are typical of the current sources and switching circuits that are found in the prior art, but none of the references found in the prior art show or suggest the invention as claimed herein.